Most of these plugs will have jumpers to allow connecting more than one of them to the same port (as I2C bus). So within the limits of power consumption, I2C bus lengths / loading, and processing bandwidth, this should allow for quite a bit of extensibility.

The PWR/I2C header has the same pinout as the BlinkM, but note that PWR is usually +5V or more (the voltage regulator input), while the SCL and SDA signals need to use +3.3V levels to match the ATmega.

Ah, yes, a mood light (orb?) which reacts to some external status would be really neat. Good example of how wireless can make it easier to spread and use status info all around the house. Now if only we could harvest enough energy somehow to make JeeNodes and everything attached to them run on thin air!